Accurate Chromosome Segregation at First Meiotic

Genetics: Early Online, published on July 27, 2016 as 10.1534/genetics.116.189217
Accurate chromosome segregation at first meiotic division requires AGO4, a
protein involved in RNA-dependent DNA methylation in Arabidopsis thaliana
Cecilia Oliver1, Juan Luis Santos and Mónica Pradillo
Departamento de Genética, Facultad de Biología, Universidad Complutense de Madrid,
Spain 28040.
1
Present address: Institute of Human Genetics, UPR 1142 CNRS, Montpellier, France
34396.
1
Copyright 2016.
Running title:
AGO4 role during Arabidopsis meiosis
Key words:
AGO4
Arabidopsis thaliana
Centromere
Meiosis
RdDM
Corresponding author:
Mónica Pradillo
Mailing address: Departamento de Genética, Facultad de Biología, Universidad
Complutense de Madrid, José Antonio Nováis 12, Madrid, Spain 28040.
Phone number: +34 913944764
Email address: pradillo@bio.ucm.es
2
ABSTRACT
RNA-directed DNA methylation (RdDM) pathway is important for the transcriptional
repression of transposable elements and for heterochromatin formation. Small RNAs are
key players in this process throughout a feedback with both DNA and histone
methylation. Taking into account that methylation underlies gene silencing and that
there are genes with meiosis-specific expression profiles, we have wondered whether
genes involved in RdDM could play a role during this specialized cell division. To
address this issue we have characterized meiosis progression in pollen mother cells
(PMCs) from Arabidopsis thaliana mutant plants defective for several proteins related
to RdDM. The most relevant results were obtained for ago4-1. In this mutant, meiocytes
display a slight reduction in chiasma frequency, alterations in chromatin conformation
around centromeric regions, lagging chromosomes at anaphase I, and defects in spindle
organization. These abnormalities lead to the formation of polyads instead of tetrads at
the end of meiosis, and might be responsible for the fertility defects observed in this
mutant. Findings reported here highlight an involvement of AGO4 during meiosis by
ensuring accurate chromosome segregation at anaphase I.
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INTRODUCTION
RNA-dependent DNA methylation (RdDM) confers transcriptional repression in all
sequence contexts (Matzke et al. 2009; Law and Jacobsen 2010). In this specialized
RNAi pathway, the base-pairing between 24 nt small interfering RNAs (siRNAs) and
nascent scaffold transcripts directs the DNA methylation complex to target loci (Law
and Jacobsen 2010; Matzke and Mosher, 2014). In Arabidopsis thaliana, the two
catalytic subunits of RNA polymerase IV (Pol IV), NRPD1A and NRPD1B, generate
single strand RNAs (ssRNAs) which serve as templates for RNA-dependent RNA
polymerase 2 (RDR2) to produce double strand RNAs (dsRNAs). These dsRNAs are
subsequently cleaved by DICER-LIKE3 (DCL3) into 24 nt siRNAs, exported to the
cytoplasm and loaded onto AGO4, AGO6, or AGO9 (Matzke et al. 2009; Law and
Jacobsen, 2010; Zhang and Zhu, 2011; Pikaard et al. 2012; Ye et al. 2012). Afterwards,
these complexes are imported back to the nucleus to target transcripts generated by Pol
V at the same loci, before they are released from the chromatin (Wierzbicki et al. 2008,
2009; Liu et al. 2014). Additionally to the sequence complementarity between the 24 nt
siRNAs and the nascent Pol V transcripts, Pol V subunit NRPE1 possesses an AGO4binding motif (known as the AGO hook), located in the carboxy-terminal region (ElShami et al. 2007). AGO4 is then able to recruit the DNA methyltransferase DOMAINS
REARRANGED METHYLTRANSFERASE 2 (DRM2) to establish de novo DNA
methylation (for a detailed characterization of this pathway see Bologna and Voinnet
2014; Borges and Martienssen 2015).
RdDM affects transcription of transposons and repeated DNA elements through
de novo methylation of cytosines in all sequence contexts (CG, CHG and CHH
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contexts, where H denotes A, T, or C). In this DNA methylation the methyltransferases
DRM1 and DRM2 plays a key role (Cao and Jacobsen 2002a; Henderson et al. 2010;
Law
and
Jacobsen
2010).
However,
other
methyltransferases,
such
as
CHROMOMETHYLTRANSFERASE 3 (CMT3) and DNA METHYLTRANSFERASE
1 (MET1) are involved in CHG and CG methylation maintenance, respectively (Jones et
al. 2001; Cao and Jacobsen 2002b; Cao et al., 2003). Furthermore, CHG methylation,
previously established by DRM2, is recognized by the H3K9 histone methyltransferase
KRYPTONITE (KYP), reinforcing the repressed chromatin state of methylated DNA
(Cao et al. 2003; Sasaki et al. 2012).
On these grounds, we have wondered whether genes involved in siRNA
biogenesis and RdDM could be important during meiotic division in A. thaliana. In
fission yeast and animals, the regulation of histone methylation is necessary for meiotic
recombination (Wahls et al. 2008; Acquaviva et al. 2013; Crichton et al. 2014). In
maize, the absence of proteins involved in DNA methylation leads to alterations in
sporogenesis and megagametogenesis (García-Aguilar et al. 2010). In this species,
defective mutants for AGO104 (ortholog of AGO9) display an apomixis-like phenotype,
revealing that this gene is essential during meiosis (Singh et al. 2011). In A. thaliana, it
has been reported that methylation status might influence meiotic homologous
recombination (HR). The hypomethylated ddm1 (decrease in DNA methylation 1) and
met1 mutant plants display a total number of reciprocal genetic exchanges, crossovers
(COs), similar to wild-type (WT) plants. However, they show differences in the
recombination frequency along chromosomes respect to WT plants (Melamed-Bessudo
and Levy, 2012; Mirouze et al., 2012; Yelina et al., 2012). Additionally, plants
defective for AGO9, a protein from the same phylogenetic clade than AGO4 which is
5
expressed in the germ-line, show a slight increase in the mean chiasma frequency per
cell respect to WT plants (Oliver et al. 2014). Here we present results that reveal the
influence of a protein required for RdDM, AGO4, on chromatin organization at both
centromeric and pericentromeric regions, highlighting its importance in ensuring an
accurate chromosome segregation.
MATERIALS AND METHODS
Plant materials
All the mutants evaluated are in Col background except ago4-1 which is in Ler
(Zilberman et al. 2003). ago4-2 is a dominant mutation resulting from the substitution
of a Glu at position 641 (located inside the PIWI domain) by a Lys (Agorio and Vera
2007). Mutant seeds were kindly donated by Dr. Pablo Vera (Universidad de Valencia,
Spain). The remaining mutants correspond to T-DNA insertion lines and they were
obtained from Salk Institute Genomic Analysis Laboratory and provided by the
Nottingham Arabidopsis Stock Centre (NASC) (Alonso et al. 2003). In this work, we
have analyzed the following single mutants: ago4-1, ago4-2, ago4-1t, ago6-2, dcl3-1,
kyp-4, nrpd1a-8, and nrpe1-11; and the triple mutants: cmt3-11 drm1-2 drm2-2, met1-3
drm1-2 drm2-2, and kyp-6 drm1-2 drm2-2. Plants were cultivated on a soil mixture of
vermiculite and commercial soil (3:1) and grown in a greenhouse under a 16/8 hours
light/dark photoperiod, at 18-20 with 70% humidity. Primers listed in Table S1 and
LBb1.3
(5´ATTTTGCCGATTTCGGAAC3´,
for
SALK
lines)
or
LB2
(5´GCTTCCTATTATATCTTCCCAAATTACCAATACA3´for SAIL lines) were used
for genotyping.
6
Seeds from Ler and ago4-1 (n = 215 and n = 218, respectively) were put on
plates containing GM medium to assess the germination percentage. The number of
germinated seeds in each background was evaluated 9, 11, 13, and 18 days after sowing.
Cytological analysis
Pollen viability was quantified by Alexander staining (1969) with some modifications
(Peterson et al. 2010). Fixation of flower buds, slide preparations and fluorescence in
situ hybridization (FISH) were performed according to Sánchez-Morán et al. (2001).
The following DNA probes were used: pTa71 (45S rDNA; Gerlach and Bedbrook
1979), pCT4.2 (5S rDNA; Campell et al. 1992), pAL1 (centromeric DNA repeat,
Martínez-Zapater et al. 1986), and pLT11 (telomeric DNA repeat, Richards and
Ausubel 1988). Chromosome preparations for subsequent immunolocalizations of
histone H3 modifications, centromeric histone H3 variant (CENH3), and α-tubulin were
obtained by a squash technique as described by Manzanero et al. (2000), with minor
modifications (Oliver et al. 2013). ASY1, ZYP1, RAD51 and DMC1 proteins were
immunodetected according to the spreading protocol described by Armstrong et al.
(2002) (Table S2). Secondary antibodies were FITC conjugated (1:50, Sigma) and Cy3
conjugated (1:300, Sigma). Immunolocalization of 5mC was previously described in
Oliver et al. (2014).
Sensitivity to -rays
Seeds from Ler and ago4-1 were surface sterilized in 2.5% sodium hypochlorite
solution during five minutes. After three washes in sterile water and an overnight at 4°,
the seeds were exposed to 150, 300, and 500 Gy (2.94 Gy/min) from a
7
137
Cs source
(IBL 437C CIS BIO International) and sown on GM plates. The number of true leaves
and the fresh weight of the seedlings were recorded 14 days after treatment.
qPCR
Expression analyses were performed as previously described by Pradillo et al. (2012).
Details of the primers and probes used are shown in Table S3. Fold variation was
considered over a calibrator using the ΔΔCt method (Livak and Schmittgen 2001).
Statistical analyses
Statistical analyses used in this work were managed with the software SPSS Statistics
17.0.
Table S1 contains the sequence of the primers used for genotyping. Information about
the antibodies used is provided in Table S2. Table S3 contains the sequence of the
primers used in the expression analyses and numbers corresponding to the TaqMan
probes (Roche).
RESULTS
Plant fertility and seed germination in mutants of genes related to RdDM
We have analyzed the following single mutants of genes related to RdDM: ago4-1
(knockout, KO; Ler background), ago4-1t, ago4-2 (knockdown, KD; Col background),
ago6-2, dcl3-1, kyp-4, nrpd1a-8 (defective for one of the catalytic subunits of Pol IV),
8
nrpe1-11 (defective for the largest subunit in Pol V). We have also examined triple
mutants defective for different methyltransferases: cmt3-11 drm1-2 drm2-2, met1-3
drm1-2 drm2-2, and kyp-6 drm1-2 drm2-2. Only the single mutant ago4-1 and the triple
mutant met1-3 drm1-2 drm2-2 showed reduced fertility. The semisterility of the triple
mutant may be explained by defects in gametogenesis and embryo-viability (Saze et al.
2003; Xiao et al. 2006). However, in ago4-1 we detected interplant variation in the
number of non-viable pollen grains (2-21%), suggestive of meiotic defects (five flowers
analyzed; Figure S1), and also a reduction in the percentage of germinated seeds respect
to WT plants: 46.38% vs. 98.13% (18 days after sowing; t = 14.77; p < 1 x 10-3; Figure
S2). Furthermore, a proportion of ago4-1 seeds (20.4%) showed smaller size and
dehydrated appearance, 2.3% presented two root apical meristems (RAMs), and 0.9 %
did not show any RAM (n = 216, Figure S2).
Cytological analysis of meiosis
Pollen mother cells (PMCs) from ago4-1 displayed the highest number of meiotic
alterations among all mutants analyzed, namely: i) A different chromatin compaction at
centromeric and pericentromeric regions at pachytene (Figures 1A, 1C). In WT PMCs
there are two strongly DAPI stained regions per bivalent corresponding to
pericentromeric regions that flank the centromere, which is faintly DAPI stained (Figure
1B). However, in ago4-1 centromeric and pericentromeric regions were not clearly
distinguished because they showed a similar DAPI staining intensity (Figure 1D); ii)
Chromosome decondensation from diplotene to metaphase I (Figures 1G, 1H, 1K). 47%
of meiocytes at metaphase I (n = 121) showed this feature (Table 1). In addition, some
bivalents displayed centromeric regions with forked appearance (Figure 1L); iii)
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Presence of interchromosomal bridges (Figure 1O), lagging chromosomes (Figure 1P),
and occasional fragments at anaphase I (Figures 1S, 1T). The percentage of these cells
was around 30% (n=26). However, this percentage could be overestimated, since
defects on chromosome segregation might increase the duration of this stage; iv)
Presence of chromatin accumulations and isolated chromosomes at late telophase II.
These defects are responsible for the formation of polyads in 47.7 % of cells analyzed (n
= 300; Figures 1W, 1X).
The mutant ago4-2 showed meiotic alterations similar to those described in
ago4-1, although in minor extent: slight differences in the conformation of centromeric
and pericentromeric regions at pachytene respect to WT; chromosome decondensation
from diplotene to metaphase I, exhibiting 27% of decondensed metaphases I (n = 177;
Table 1; Figures 1I, 1M); interchromosomal bridges and lagging chromosomes at
anaphase I that originate the abnormalities observed at metaphase II (Figures 1Q, 1U);
and 3% of polyads (n = 300; Figure 1Y). Female meiosis was apparently normal in
both ago4 mutants (Figure S3), but the results are not conclusive due to the low number
of cells analyzed.
The mutants dcl3-1, ago6-2, nrpe1-11, kyp-4, and kyp-6 drm1-2 drm2-2 also
displayed chromatin decondensation in a percentage of diplotene-metaphase I PMCs
(Table 1; Figure S4). Interchromosomal anaphase I bridges were observed in ago6-2,
kyp-6 drm1-2 drm2-2, and kyp-4 (Figures S4F-S4H), but, in contrast to ago4-1 and
ago4-2, chromosome fragments and isolated chromosomes were absent. Finally,
meiosis seems to be cytologically normal in nrpd1a-8, ago4-1t (mutant with the T-DNA
insertion into the promoter), cmt3-11 drm1-2 drm2-2, and met1-3 drm1-2 drm2-2.
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Chiasmata were scored according to previously established criteria (SánchezMorán et al. 2001, 2002). Between one and three plants per mutant were analyzed to
estimate mean cell chiasma frequencies at metaphase I. Only three mutants showed
significant differences in this parameter respect to WT plants: ago4-1 and ago4-2
exhibited a significant decrease (t = 2.92; p = 4 x 10-3, and t = 3.11; p = 2 x 10-3,
respectively), whereas nrpd1a-8 presented a significant increase (t = 2.44; p = 16 x 103
). The remaining mutants analyzed displayed a general tendency toward a slight,
although non-significant, increase of this parameter (Table 2). At chromosome level,
there was a significant increase of chiasma frequency in the short arms of chromosomes
2 (nrpd1a-8, kyp-4, ago6-2, met1-3 drm1-2 drm2-2, dcl3-1, and ago4-2) and 4 (nrpd1a8, ago6-2, dcl3-1, and ago4-2). By contrast, ago4-1 and ago4-2 showed a decrease in
chiasma frequency in both chromosome 1 and the long arm of chromosome 2.
Regarding chromosome 3, there was a significant reduction in the short arm (ago4-1)
and in the long arm (ago6-2, nrpe1-11, cmt3-11 drm1-2 drm2-2, and ago4-2). Finally,
met1-3 drm1-2 drm2-2, ago4-2, and nrpe1-11 displayed a decrease in the long arm of
chromosome 5.
Since the most relevant results obtained from a meiotic point of view are those
referred to ago4 mutants, particularly ago4-1, we decided to perform a more accurate
study based on the following issues: i) characterization of centromeric and
pericentromeric regions; ii) histone modifications during meiosis; iii) synapsis and HR;
and iv) chromosome segregation. Additionally, we have analyzed possible defects in
mitosis and DNA repair.
Characterization of centromeric and pericentromeric regions
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As mentioned before, centromeric and pericentromeric regions of ago4-1 and ago4-2
displayed a different DAPI staining respect to WT, especially at pachytene. To gain
insight into this phenomenon, we performed a FISH using the centromeric sequence of
180 bp (pAL1), and a telomeric probe (PLT11) as a positive control. The overall size of
the centromeric signals was conspicuously smaller in the ago4 mutants than in WT
plants, while no apparent differences in the size of the telomeric signals were observed
(Figure 2). We also detected differences among the chromosomes.
We performed an immunodetection of 5-methyl cytosine (5mC), because
heterochromatic regions in plant chromosomes are usually enriched in this DNA
modification. In Col, Ler and ago4-2, 5mC was restricted to pericentromeric regions
(Figures 3A-3D), but in ago4-1 it was located at both pericentromeric and centromeric
regions (Figures 3E-3H). Additionally, both mutants displayed a normal distribution of
CENH3, even in meiocytes with abnormal chromosome segregations (Figures S5-S7).
Analysis of histone modifications during meiosis
Dimethylated H3K9 (H3K9me2) is the main histone H3 modification regulated by
RdDM and it is located at heterochromatic pericentromeric regions. We did not detect
any variation in the distribution of this histone modification between ago4 and WT
plants (Figure S8). The distribution was also similar for the following euchromatic H3
modifications (Oliver et al. 2013): H3K4me2, H3 acetylated (H3Ac) and H3K27me3
(Figures S9-S11). We have also analyzed the chromosome distribution of H3S10Ph, a
modification associated with condensation in mammals (Cobb et al. 1999) and with the
process of sister chromatid cohesion in plants (Kaszas and Cande 2000; Manzanero et
al. 2000). In A. thaliana, this post-translational modification appears at diplotene-
12
diakinesis and remains until the beginning of anaphase I. It reappears again at
metaphase II to be finally absent at anaphase II (Oliver et al. 2013). This pattern was
broadly similar in ago4-1, ago4-2 and WT pachytene PMCs (Figure 4; Figure S12),
although a slight delay in H3S10Ph disappearance was observed at anaphase I in ago4-1
(Figure 4). This delay was probably associated to chromosome regions involved in
interchromosomal bridges (Figures 4A-4L). The pattern of appearance-disappearance at
second meiotic division was similar to that observed in WT plants (Figures 4M-4T).
Synapsis and homologous recombination (HR)
Since ago4-1 and ago4-2 were the only mutants that showed a significant reduction in
the mean cell chiasma frequency as compared to WT plants, we decided to analyze HR
and synapsis by means of the immunolocalization of different proteins. The
recombinases RAD51 and DMC1 were loaded normally onto meiotic axes and synapsis
progressed correctly according to the detection of ASY1 and ZYP1, proteins associated
to the lateral and central elements of the synaptonemal complex (SC), respectively
(Figure S13).
In ago4-1 we have also analyzed the expression of some representative genes
involved in HR: SPO11-1, ATM, ATR, BRCA1, BRCA2B, RAD50, RAD51, RAD51C,
DMC1, MSH4, MLH3, MUS81, SMC6A, and SMC6B. In bud samples (enriched in
meiocytes) only SPO11-1 (2.52-fold), and SMC6B (2.36) were over-expressed, while
DCM1 (4.43), and MUS81 (2.11) only were in leaf samples (Figure S14). ATR (0.40),
RAD51C (0.35), and SMC6B (0.33) were under-expressed in leaf samples.
Meiotic chromosome segregation
13
To assess whether abnormalities in chromosome segregation at anaphase I observed in
ago4-1 were related to alterations in the structure and/or function of the spindle, we
examined this structure by α-tubulin immunolocalization. The most intriguing finding
was the presence of microtubule bundles with an altered disposition, located in
transversal orientation with respect to the division axes at anaphases I and II (Figure 5).
The immunodetection of α-tubulin in the polyads revealed that some of the four pollen
grains originated were multinucleate (Figure S15). However, we did not detect
abnormal mature pollen grains with more than three nuclei. Hence, it is feasible to think
that irregular pollen grains observed might degenerate before the occurrence of pollen
mitoses (Figure S16).
Mitosis and sensitivity to γ-rays
Simultaneously to meiotic characterization, a cytological analysis of mitosis in ago4-1
and ago4-2 was conducted. Prophase was apparently normal, but we observed
associations between decondensed chromosomes at metaphase (36%; n = 50),
anaphases with delay chromatid segregations (36% in ago4-1, 32% in ago4-2; n = 50)
and interchromatid bridges (5% in both mutants; n = 50). However, telophases were
apparently normal (Figure 6).
Due to the presence of these mitotic defects we decided to evaluate possible
deficiencies in DNA repair by irradiating seeds of ago4-1 and Ler with γ-rays. This
DNA damaging agent has a high mutagenic potential in plants and produces a large
number of lesions which mainly generate double-strand breaks (DSBs). Among the
different doses of irradiation, only at 450 Gy the mutant showed a significant decrease
14
respect to WT plants in both the number of leaves and the fresh weight per plant (t =
2.53; p = 13 x 10-3; Figure S17).
DISCUSSION
The semisterility displayed by ago4-1 had only been associated to developmental floral
organ defects (Zilberman et al. 2004). However, our results suggest that it can also be
related to the formation of unviable pollen grains (Figure S1). Furthermore, ago4-1
seeds showed a germination delay (Figure S2) and a considerable variability at
morphological level, similar to that observed in superman (sup) mutants, defective for a
zinc finger protein (Gaiser et al. 1995). Indeed, AGO4 is involved in silencing the SUP
gene (Zilberman et al. 2003).
Centromeric and pericentromeric regions
Among all mutants analyzed, the most conspicuous meiotic alterations were observed in
ago4 mutants (Figure 1), especially in ago4-1 PMCs. Observations reported here
suggest that differences in chromatin organization at both centromeric and
pericentromeric regions respect to WT could play a role in this scenario (Figures 2 and
3). The main components of Arabidopsis centromere are a sequence of 180 bp in
thousands of tandemly repeated copies and transposons like Athila, Tat, Tim or Copia.
Only 15% of 180 bp sequences are connected to CENH3 (Nagaki et al. 2003; Schubert
et al. 2012). In contrast to the centromere, pericentromeric regions can show different
condensation levels (Schubert et al. 2012). These regions have a low gene density and
are rich in transposons and repeated DNA sequences (May et al. 2005). In ago4-1 the
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signal size corresponding to the 180 bp sequence observed by FISH was considerably
smaller than in the WT (Figure 2). This reduction in DNA FISH signals could reflect a
partial centromeric deletion, although changes in chromatin conformation could also
difficulty the accessibility of the probe to the complementary DNA sequence. However,
apparent changes in the location pattern of CENH3, required to kinetochore assembly,
were not detected (Figures S5-S7). Zilberman et al. (2003) reported that ago4-1 does
not affect methylation levels at the 180 bp sequence, but we have observed a decrease in
5mC at pericentromeric regions, associated with a punctate pattern at centromeres
(Figure 3). This anomalous 5mC distribution could be related to the alterations detected
during anaphase I (Figures 1, 3 and 5). Mutations in fission yeast of genes coding for
proteins involved in siRNA pathway produce abnormalities in chromosome segregation,
and also a loss of pericentromeric heterochromatin silencing detected by a decrease in
the signal corresponding to H3K9me2 (Volpe et al. 2003; Fukagawa et al. 2004;
Kanellopoulou et al. 2005). However, we have not detected dissimilarities in the
distribution pattern of this and others H3 modifications in ago4 mutants (Figures S8S11).
Chromatin architecture
The final effect of RdDM is methylation of DNA that is determinant in chromatin
compaction (Liu et al. 2016). Indeed, siRNAs are involved in the recruitment of
chromatin modification complexes that lead to the formation of heterochromatin
(Wassenegger, 2005; Matzke and Mosher, 2014). General chromatin decondensation
observed from diplotene to metaphase I was a common feature in the mutants of genes
involved in RdDM, although the percentage of decondensed nuclei was variable among
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them. The highest percentages corresponded to ago4-1 and nrpe1-11 (Table 1; Figures 1
and S4). However, we did not observe differences between ago4-1 and WT PMCs in the
distribution pattern of H3Ac, despite histone deacetylase 6, HDA6, is necessary for the
propagation of de novo methylation directed by RNA (Aufsatz et al. 2002). Likewise,
we did not detect differences for other H3 post-translational modifications (Figures S8,
S9 and S11). These results concur with those reported in other mutants that show
alterations in the siRNA machinery (Naumann et al. 2005; Pontes et al. 2009).
Homologous recombination
Drosophila mutants defective in piRNA/ra-siRNA components do not show important
changes in CO frequency (Cross and Simmons, 2008). In A. thaliana, we have found
only three mutants for genes involved in RdDM with significant variations in the mean
cell chiasma frequency respect to WT plants: ago4-1 and ago4-2 showed a decrease
while nrpd1-8 displayed an increase (Table 2). The results obtained here also reveal that
the same bivalent, and their chromosome arms, may behave in a different way in
different mutants (Table 2). This indicates the existence of factors controlling meiotic
recombination at arm/chromosome level that are differentially regulated in the mutants.
In this sense, met1 and ddm1 mutants present a reduction in DNA methylation at
heterochromatic pericentromeric regions and changes in CO frequency along
chromosomes, although their overall chiasma frequency is similar to that observed in
WT plants (Melamed-Bessudo and Levy, 2012; Mirouze et al. 2012; Yelina et al.
2012).
Maintenance of chromosome architecture is important for the correct function of
meiotic proteins. Nevertheless, and despite the reduction in CO frequency, ago4-1 and
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ago4-2 showed full synapsis and the recombinases RAD51 and DMC1 were normally
loaded onto chromatin (Figure S13). Furthermore, expression analysis of genes involved
in meiotic HR did not reveal any clear tendency (Figure S14). Additionally, ATR and
RAD51C and SMC6B are under-expressed in the somatic line of ago4-1 (Figure S14),
despite the mutant is not hypersensitive to γ-rays (Figure S17). Similar results have been
found in other mutants affected in siRNA biogenesis in which there is a low
spontaneous HR frequency and DNA repair enzymes are not overexpressed (Wei et al.
2012; Yao et al. 2016).
Chromosome segregation
ago4 mutants exhibited some abnormalities at mitotic anaphases, although telophases
were normal (Figure 6). They also displayed defects during meiosis, especially at
anaphase I, lying in the presence of interchromosomal bridges and a delay in
chromosome segregation (Figure 1). These abnormalities were also observed, although
in minor extension, in nrpe1-11, kyp-4 and kyp-6 drm1-2 drm2-2 (Figure S4).
Chromosome bridges at anaphase I have been described in mutants defective in the
repair of programmed DSBs. In these mutants, the bridges are mostly accompanied by
chromosome fragments that are detected at late prophase I (Schommer et al. 2003;
Puizina et al. 2004; Wang et al. 2012). However, in ago4-1 chromosome fragmentation
was almost nominal and restricted to anaphase I (Figures 1 and 4).
In A. thaliana H3 phosphorylation occurs all along the chromosomes from
diplotene to telophase I, and from late prophase II to metaphase II, disappearing at
telophase II (Caperta et al. 2008; Oliver et al. 2013; Figure S12). Manzanero et al.
(2000) and Kaszas and Cande (2000) have suggested that in plants H3 phosphorylation
18
is related to sister chromatid cohesion. Thus, it is tempting to speculate that the
association of H3S10ph to bridges and chromosome fragments at anaphase I could
reflect the difficulties in the separation of sister chromatids (Figure 4). Other actors in
this scenario could be members of the Aurora family (Demidov et al. 2005) that are
responsible for the cell-cycle dependent phosphorylation of H3 at serine 10 (Demidov et
al. 2009), and for ensuring correct chromosome segregation (Demidov et al. 2014). In
addition, depletion of Aurora B in Drosophila, Caenorhabditis elegans and results in
reduced H3S10ph and is related to defects in chromosome segregation (Wei et al. 1999;
Adams et al. 2001; Giet and Glover 2001). Also, a mutation of H3S10 in Tetrahymena
produces segregation defects (Wei et al. 1999). In Arabidopsis meiosis, the alterations
in the dynamics of this modification observed in ago4-1 could be responsible for the
improper spindle formation (Figure 5). Alterations in chromatin condensation, failures
in spindle morphogenesis and polyad formation were also observed in the ago104 maize
mutant, and in the Arabidopsis mutants radially swollen 4 (rsw4) and tardy
asynchronous meiosis 1 (tam1), defective for a separase and a cyclin, respectively.
These mutants also show delayed chromosome segregation (d´Erfuth et al. 2010; Singh
et al. 2011; Yang et al. 2011). This phenomenon, although less extreme, is also
displayed by defective mutants for CENH3 (Ravi et al. 2010; Lermontova et al. 2011).
In mammals, defects in spindle formation and chromosome alignment have also been
detected in the female meiosis of mutants defective for endogenous siRNAs (Stein et al.
2015).
In conclusion, the changes in the organization of centromeric and
pericentromeric chromatin observed in ago4 are probably the origin of the failures
observed in spindle organization. Alterations in kinetochore-microtubule interactions
19
could be responsible for chromosome segregation defects, manifested cytologically as
chromosome bridges and fragments. Cytokinesis at the end of the meiotic process
would lead to the formation of four meiotic aberrant products. Altogether these results
suggest that AGO4 may possess a meiosis-associated cellular function that seems to be
independent of other proteins also involved in RdDM. In this sense, recent publications
have highlighted the existence of siRNAs generated via an alternative route independent
of DCLs (siRNAs independent of DCLs, sidRNAs). These sidRNAs are associated with
AGO4 and capable of directing DNA methylation, playing key roles in the initiation of
RdDM (Ye et al. 2015). Additionally, although in general AGO6 is redundant with
AGO4 in RdDM, they can play non-redundant roles in regulating the same RdDM
target and may act sequentially to mediate siRNA-guided DNA methylation (Duan et al.
2015). It could explain the differences in the mutant phenotypes that we have analyzed.
Although further studies should be needed to decipher the detailed mechanism how
AGO4 influences Arabidopsis meiosis, this study marks the way.
ACKNOWLEDGMENTS
This work has been supported by grants from European Union Framework Program 7
(Meiosys-KBBE-2009-222883) and Ministerio de Economía y Competitividad of Spain
(AGL2012-38852). We thank Dr. Tomás Naranjo (Universidad Complutense de
Madrid, Spain), Dr. Andreas Houben (IPK, Germany), and Dr. Chris Franklin
(University of Birmingham, UK) for providing antibodies used in this work. ago4-2
seeds were kindly donated by Dr. Pablo Vera (Universidad de Valencia, Spain).
20
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30
Table 1 Metaphases I with decondensed chromosomes.
MUTANT
METAPHASES I WITH
DECONDENSED
CHROMOSOMES (%)
n
kyp-4
4
137
ago6-2
6
109
kyp6-1 drm1-2 drm2-2
9
66
dcl3-1
22
105
ago4-2
27
177
nrpe1-11
44
71
ago4-1
47
121
n: number of cells analyzed.
31
Table 2 Mean chiasma frequencies per cell, per bivalent and per bivalent arm (short vs.
long) in Ler, Col and mutants analyzed in this study.
BIVALENTS
1
Ler
ago4-1
Col
nrpd1a-8
kyp-4
ago6-2
kyp-6
drm1-2
drm2-2
met1-3
drm1-2
drm2-2
dcl3-1
nrpe1-11
cmt3-11
drm1-2
drm2-2
2
3
4
5
s
l
s
l
s
l
s
l
s
l
-
-
0.56
1.02
0.97
1.00
0.52
0.95
0.86
1.16
2.35
(0.25)
1.57
(0.17)
1.97
(0.21)
1.48
-
-
0.62
0.90*
0.74***
1.05
0.60
2.13*
(0.24)
1.52
(0.17)
1.80*
(0.20)
1.48
-
-
0.61
1.14
0.90
1.26
0.48
2.52
(0.25)
1.75
(0.17)
2.16
(0.21)
1.49
-
-
0.78*
1.10
0.97
1.17
0.67*
2.63
(0.25)
1.88
(0.18)
2.13
(0.20)
1.72*
-
-
0.80*
1.11
0.97
1.13
0.63
2.51
(0.24)
1.91
(0.18)
2.11
(0.20)
1.71*
-
-
0.80*
1.12
0.86
1.10*
0.74**
2.34
0.23()
1.92
(0.19)
1.96*
(0.19)
1.90**
-
-
0.76
1.13
0.97
1.13
0.52
2.37
(0.23)
1.90
(0.18)
2.18
(0.21)
1.66
-
-
0.79*
1.09
0.97
1.21
0.56
2.35
(0.23)
1.88
(0.18)
2.18
(0.21)
1.71
-
-
0.80*
1.07
0.92
1.12
0.69**
2.43
(0.24)
1.87
(0.18)
2.04
(0.20)
1.72*
-
-
0.75
1.15
0.91
1.11*
0.61
2.39
(0.24)
1.91
(0.19)
2.02
(0.20)
1.71*
-
-
0.62
1.04
0.94
1.10*
0.60
2.42
(0.24)
1.65
(0.17)
2.04
(0.21)
1.62
-
-
0.78*
0.97**
0.86
1.04**
0.67*
(0.16) 2.02
0.88
0.84
(0.17) 1.91
1.01
0.97
(0.15) 2.28
1.05
1.00
(0.16) 2.32
1.08
1.00
(0.16) 2.31
1.16
1.00
(0.18) 2.22
1.13
0.96
(0.16) 2.24
1.15
(0.21)
1.07
(0.22)
1.30
(0.22)
1.32
(0.22)
1.31
(0.22)
1.22
(0.21)
1.28
(0.22)
1.00
1.12*
(0.17) 2.12
(0.21)
1.03
0.96
(0.17) 2.21
1.11
1.25
(0.22)
0.98
1.12**
(0.17) 2.11*
(0.21)
1.02
0.96
(0.16) 2.15
1.03
0.99
1.19
(0.22)
1.10**
C
n
9.38
63
8.84** 90
10.20 69
10.68* 60
10.53 75
10.34 50
10.34 67
10.29 34
10.27 75
10.14 66
9.88
52
9.61** 69
2.17*** (0.23)
1.75 (0.18) 1.90** (0.20)
1.70* (0.18) 2.09* (0.22)
C: mean cell chiasma frequency; n: number of cells; s: short arm; l: long arm. The values in parentheses are
the bivalent chiasma frequencies as proportions of the total cells. *, p < 0.05; **, p < 0.01; *** p < 0.001.
Chromosome 1 is considered as a whole because it is not possible to distinguish chromosome arms.
ago4-2
32
Figure 1 Meiotic stages in PMCs from Ler, ago4-1, and ago4-2. (A, B, F, J, N, R, V)
Representative images from Ler PMCs. (C, D, G, H, K, L, O, P, S, T, W, X)
Representative images from ago4-1 PMCs. (E, I, M, Q, U, Y) Representative images
from ago4-2 PMCs. (A, C, E) Pachytene. (B) Enlarged centromeric and pericentromeric
regions from A. (D) Enlarged centromeric and pericentromeric regions from B. (F-I)
Diplotene. (J-M) Metaphase I. (N-Q) Anaphase I. Red arrows indicate a chromosome
bridge (O), delayed segregation of homologous chromosomes (P), and lagging
chromosomes (Q). (R-U) Metaphase II. Red arrows indicate laggards in S, T and U. (VY) Tetrads. Red arrows indicate chromatin accumulation in W and some micronuclei in
X and Y. Bars = 5 µm. (Z) Analysis of chromosome condensation at metaphase I and
polyad formation in ago4-1 and ago4-2. Differences were observed in both mutants
compared to the respective WT backgrounds. *, p < 5 x 10-2; **, p < 10-2; ***, p < 10-3.
33
Figure 2 FISH to detect centromeres (pAL1) and telomeres (PLT11) at pachytene.
(A, D) WT. (B, E) ago4-1. (C, F) ago4-2. Centromeres are showed in green and
telomeres in red. White arrows indicate pAL1 signals. Bars = 5 µm.
34
Figure 3 Immunolocalization of 5-methyl cytosine in PMCs from WT and ago4-1
plants. (A, B) Ler. (C, D) Details of A and B. (E, F) ago4-1. (G, H) Details of E and F.
Regions of the further enlarged pictures are indicated. White arrows point out
centromeres. Bars = 5 µm.
35
Figure 4 Immunolocalization of H3S10Ph in ago4-1. (A-I) Anaphases I. Arrows
indicate bridges or chromosome fragments. (J-O) Metaphase II. (P-R) Polyad. Bars = 5
µm.
36
Figure 5 Immunolocalization of α-tubulin and CENH3 in PMCs from Ler and
ago4-1. (A-H) WT. (I-T) ago4-1. (A-D, M-P) Anaphase I. The white arrow indicates a
microtubule bundle in opposite orientation from the spindle. (E-H, Q-T) Anaphase II.
(I-L) Metaphase I. The white arrow indicates a microtubule bundle in opposite
orientation from the spindle, depicted by a double-headed arrow. Bars = 5 µm.
37
Figure 6 Cytological analyses of mitosis in somatic cells from ago4-1. (A-D) WT.
(E-H) ago4-1. (I-L) ago4-2. (A, E, I) Prophase. (B, F, J) Metaphase. (C, G, K)
Anaphase. Arrows indicate a lagging chromatid (G) and an interchromatid bridge (K).
(D, H, L) Telophase. Bars = 5 µm. (M) Cells were scored to analyze the decondensation
at metaphase, delays in chromatid segregation, and presence of interchromatid bridges
at anaphase. There were statistical differences respect to WT in chromosome
condensation at metaphase (ago4-1) and anaphase alterations (ago4-1 and ago4-2). *, p
< 5 x 10-2; **, p < 10-2; ***, p < 10-3.
38